The present invention relates to a liquid monitoring apparatus, and particularly to a lubricating oil monitor for internal combustion engines.
Internal combustion engines all require the presence of a lubricating oil to appropriately lubricate the moving parts including the piston and crankshaft bearings, and the like. Proper lubrication requires maintaining a minimum amount of lubricating oil in the crankcase sump. In most internal combustion engines, a dip stick unit is releasably mounted to the engine block and projects downwardly into the oil sump. The user can readily check the level of the oil in the sump by removing and viewing the extent of the oil on the dip stick. In pressurized lubricating systems, a pressure sensing switch is also conventionally mounted in the lubricating flow system. The pressure level will drop if the oil decreases below a selected safe operating level. The switch is connected into a suitable indicating system such as a gauge, light or the like to establish a readout and warning to the user whenever the oil level drops below a minimum level. In smaller internal combustion engines, pumps and associated pressurized systems may not be used. Rather, a splash lubricating system is used wherein a splash arm member on the crank unit passes through the oil in the sump and throws the oil into the operating components to provide the necessary lubrication. The conventional pressure responsive protection system is not therefore applicable to such lubrication systems.
Small internal combustion engines are often used in lawn mowers, lawn tractors and other relatively small applications. Operators of such devices often fail to adequately maintain the necessary lubricating oil in the sump. Even relatively short periods of engine operation with an inadequate oil supply may result in severe damage if not complete failure of the engine. The prior art has suggested sensing systems to replace manual monitoring of the oil level as measured by a dip stick. The prior art has also suggested the use of an automatic protective system including a sensor coupled into the engine oil sump or bath which will either shut-down operation of the engine, operate a visual or audio signal, or both, should the oil level drop to an inadequate level.
Mechanical sensor systems have been suggested for use with such splash lubrication systems. Others have suggested various electrical sensing systems. Generally, the prior art has used various float switches, capacitor switches, thermoswitches, and spark gap units located to produce a change in an electrical output with the oil level below the selected minimum level in the crankcase.
In spark gap systems, oil which is a relatively poor electrical conductor, immerses a pair of electrodes so long as the oil level is at or above the selected and safe level. The electrodes are connected into the ignition system as a source of voltage. If the oil drops below the safe level, the electrodes are exposed and conduction is permitted.
For example, U.S. Pat. Nos. 2,123,460 and 2,123,400, both of which issued on July 12, 1938 to Burschker, disclose a sensor incorporated into the oil supply tube for introducing the sensor into the oil-level checking dip tube of the engine. The sensor includes a spark gap unit mounted within the corresponding dip tube, which is grounded to the engine block. For example, a single sensing electrode is mounted in insulated relationship within the tubular dip stick which is grounded to the engine and forms the opposite sensing electrode. The sensing electrode is connected into the ignition system and is maintained at a relatively high voltage sufficient to breakdown the air between the electrode and the tubular dip tube and establish a current flow. With the oil level above a selected minimum safe level, the oil bridges the gap between the outer tube and the inner sensing electrode. The oil is a relatively good electrical insulator and prevents breakdown of the gap and conduction of current. When the oil level drops below the selected level, the high voltage establishes a current flow which is interconnected to short circuit the ignition circuit, and thereby stop engine operation instantaneously.
U.S. Pat. No. 4,256,069 which issued Mar. 17, 1981, discloses a float-operated system in which a magnetic float in a sump moving relative to a hermetically enclosed reed switch in accordance with the oil level in the sump. With a minimum oil level, the float is raised to close the switch and permit ignition. If the level drops below such level, the switch opens and prevents ignition. The detector is enclosed within a special housing within the engine sump to shield the detector from the effects of the oil splashing bars.
U.S. Pat. No. 2,529,775 which issued Jan. 20, 1948, to Maddox discloses a pressure responsive electrical switch unit having a pair of spaced electrodes. A float ball, formed of a conductive material, is gravity biased into a engagement with the electrode to complete a circuit. When the engine is started and the oil level is above a selected minimum safe level, the oil pressure increases, and lifts the ball from its normal circuit closing position thereby opening the circuit and allowing normal ignition. If the oil level drops, the ball returns to its normal seating position and contemplates the short circuit, and thereby stops engine operation. U.S. Pat. No. 4,203,408 which issued May 2, 1980, to Yanagushirt et al discloses another pressure responsive unit operating a special diaphragm type actuator. Such systems cannot operate in the usual "splash" lubricating system.
These and similar suggestions are available for automatic monitoring of the oil level. Such systems have not found wide acceptance and application in commercially available engines of the splash lubrication system. Although, the cost associated with automatic sensors may often be justified, the accuracy and reliability of available sensors, as well as possible erroneous response appears to have significantly eliminated any significant commercial application, particularly in small engines using a "splash" lubrication system.
Further at stand still and with the engine shut down, the oil level does provides an accurate indication of the adequacy of the oil supply if the engine is in a appropriate level position to locate the oil relative to the sensor such as the dip stick. The dip stick is normally applied to the side of the engine. If the engine is significantly tilted either laterally or longitudinally with respect to the position of the dip stick, the oil level in the sump will flow to a corresponding low point. This may change the apparent level at the dip stick. In a inclined or tilted position, the oil level may therefore obviously indicate that an inadequate supply is available. Generally, the user is instructed to check the oil level with the engine in an appropriate horizontal level position. If an automatic sensor is inserted into the system with the dip stick unit, an erroneous indication may be generated whenever during use, the engine is tilted as a result of the movement of the engine with the vehicle on an inclined surface.
During actual engine operation of an engine on a lawn mower or other moving vehicle, the oil bath is also in constant motion as a result of the movement of the vehicle and the spash bar. The movement of the vehicle will create an inherent motion of the oil bath in the sump often creating a wave-type motion.
The usual vibration associated with engine operation will also cause some oil movement. In addition in a splash-type lubrication system, a splash bar secured to the engine crank may move the oil from the sump into the engine parts to create the necessary lubrication, and thereby creates further agitation and motion of the oil.
Even with the oil level in the engine within the normal safe limits as indicated for example on the dip tube, the motion of the oil, with respect to the automatic sensor particularly in "splash" lubricated engines will often create a momentary indication adjacent the sensor of a low level condition. Although the condition may exist for relatively short periods of time, the sensors will often respond to give an erroneous indication. Where the sensor is merely connected to provide a visual indication no significant problem may be created and the operation may be acceptable. The sensor is however desirably interconnected as shown in the above prior art to automatically stop the engine. In most systems, the cut-off unit provides for automatic restart upon resetting of the oil with respect to the sensor. Alternatively, a manual reset may be required by the operator upon automatic shutdown. In either event, erratic operation of the sensor would at the very least be annoying to the operator as well as economically undesirable where the engine is installed in a commercial profit making operation. The resulting erratic engine operation under such condition will almost universally be unacceptable.
The inventors have realized that the basic difficulty with the prior art systems resides in attempting to measure the oil level whereas in fact the measurement so far as possible should be the volume of oil in the engine. Further, in monitoring the volume of oil, it is highly desirable to monitor relatively small changes in the volume, particularly at and around the selected safe oil volume for establishing minimum desired lubrication of the engine. The wave motion and "splash" lubricating action associated with the oil bath during system operation particularly create difficulty in the use of the oil level as a reference, or measuring of the volume of oil present.
In addition, sensors having a switch connected in the high voltage ignition system may create other practical problems. The connection of the one electrode in a high voltage system requires that a reliable interconnection be maintained in the severe vibration environment associated with the engine as well as the probable presence of large volumes of foreign matter in the surrounding air. Thus, the connection must be physically rugged. In addition, the connection must anticipate the accumulation of foreign matter on the outer surface of the electrode mounting which would tend to create a bypassing current from the electrode directly to the engine ground. The system should of course, for optimum system operation, provide accurate and similar indication of the oil supply independent of the physical orientation of the engine within the normal operating conditions. Thus as applied to a vehicle, lawn mower or other device adapted to diverse a ground area, the same indication should be given as long as the implement is in appropriate operative ground engagement. However, it is desirable that in the event of undesirable tipping of the engine for example to one side or both, that automatic stopping of the engine will be created.
Finally, the cost of the system must be maintained at a reasonable level particularly for mass production required for mass produced relatively small "splash" lubricated engines.